Term
Baseline
The line across the very back of the court. You serve from behind it.
The line at the very back of the court, running parallel to the net (3.A.4.a). You serve from behind this line (3.A.4.g, 7.A.2, Serve diagonally).
See it in a figure
This figure is from the rule "Court size and lines".
How to read the sources: numbers like (8.F.4) are rulebook sections, and links are outside sources. An "Author's note" is the author's personal experience, not an official rule.